Substraction and charge quantity generation charge transfer device

ABSTRACT

The storage electrodes are electrically connected at a point. A reference charge quantity is stored beneath the first storage electrode when the point is at a constant potential and before charges pass beneath the second storage electrode. When a charge quantity arrives beneath the second storage electrode and when the point is floating, the surface potential beneath the first storage electrode is maintained constant and a charge quantity is transferred from the first storage electrode to a third storage electrode.

BACKGROUND OF THE INVENTION

The present invention relates to a subtraction and charge quantity generation charge transfer device. It also relates to an analog signal processing charge transfer system equipped with such a device.

During the processing of analog signals by charge transfer, it is often necessary to carry out subtraction between two signals. In the prior art, the charges are generally converted into a voltage and the subtraction then takes place on the voltages. A quantity of charges corresponding to the subtraction result is then reinjected into the charge transfer system.

Conversions of charges into voltages and vice versa have the disadvantage of having only limited linearity and having a difficulty reproducible gain. Moreover, the devices used for carrying out conversions and subtractions on voltages are generally cumbersome and have a high current consumption.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a device which directly carries out the subtraction on the charge quantities.

This device has the advantage of being easy to integrate into a more complex charge transfer system. It makes it possible to obtain with a high level of accuracy a quantity of charges equal to the difference between a reference charge quantity and n charge quantities. According to a preferred embodiment, this device only requires a single polarization voltage for the storage and transfer electrodes, apart from the reference voltage of the substrate, which increases its simplicity and accuracy.

The device according to the invention also ensured with a high level of precision the generation of a quantity of charges equivalent to the n charge quantities which are substracted from the reference charge quantity.

The device according to the invention comprises a first storage electrode electrically connected to n other storage electrodes at a point P, means ensuring the injection beneath the first storage electrode of the reference charge quantity prior to the arrival of the charges beneath the n storage electrodes and when point P is at a constant voltage and means which ensure that a constant surface potential is maintained beneath the first storage electrode, during the arrival of charges beneath the n storage electrodes and when point P is left floating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show:

FIGS. 1a and 1b cross-sectional views of an embodiment of the device according to the invention and two diagrams illustrating its operation.

FIGS. 2 and 3 circuit diagrams of the embodiment of the device according to the invention shown in FIG. 1a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, the same references designate the same components but, for reasons of clarity, the dimensions and proportions of the various components have not been respected.

FIG. 1a is a cross-sectional view in the charge transfer direction of an embodiment of the device according to the invention.

A silicon semiconductor substrate 1, of type P in the present embodiment, is covered with an insulating layer 2, of silicon monoxide in the present embodiment. Charge transfer and storage electrodes are arranged in alternating manner on the insulating layer 2, whereby they are positioned substantially perpendicularly to the charge transfer direction. The transfer electrodes are deposited on an insulating reinforcement 3.

As is known, this arrangement has a double function. Firstly, it ensures the continuity of the potentials created in the semiconductor and secondly it imposes a single transfer direction for the charge carriers.

The structure having the insulating reinforcement can be replaced by overdoping of the substrate by ion implantation fulfilling the same functions.

In FIG. 1a, a first storage electrode G₁ is electrically connected by a metallic lead, external to the substrate and generally in aluminium to a single storage electrode G₂. Storage electrode G₂ forms part of a charge transfer register into which are transferred the charge quantity or quantities to be subtracted from a reference charge quantity Q_(ref). FIG. 1a only shows storage electrode G₂ of said register and the transfer electrode G'₂ which precedes G₂ in the charge transfer direction indicated by an arrow.

The first storage electrode G₁ is surrounded by two transfer electrodes G₀₁ and G₀₂. Transfer electrode G₀₁ is followed by a charge injection diode D constituted by a type N⁺ diffusion. Transfer electrode G₀₂ is followed by a storage electrode G₃.

Point P can be connected to a constant voltage V_(O), but it can also be left floating, i.e. it need not be connected to any voltage source. Switch I₁ in FIG. 1a symbolically represents the two possibilities for the polarization of point P.

Grids G₀₁, G₀₂ and G₃ can be connected to constant voltage V_(O) or to a voltage below V_(O), which may have the polarization voltage of substrate V_(sub). These two polarization possibilities are symbolically represented by switch I₂ connected to G₀₁ and by switch I₃ connected to G₀₂ and G₃.

In practice, switches I₁, I₂ and I₃ are generally constituted by one or more MOS transistors.

Charge injection diode D receives a polarization voltage V_(D).

FIGS. 1a and 1b also have two diagrams illustrating the operation of the device according to the invention. These diagrams show the development of the surface potential φ_(S) in substrate 1. The hatched areas indicate the presence of minority carriers.

FIG. 1a shows the different surface potentials in the substrate at time t₁, when switches I₁, I₂ and I₃ are in the position indicated in FIG. 1a. At time t₁, point P is at constant voltage V_(O) and the surface potential beneath the storage electrodes G₁ and G₂ is the same, said potentials being V_(A1) and V_(B1).

The transfer electrode G₀₁ is at constant voltage V_(O) through switch I₂. A reference surface potential φ_(ref) is thus established beneath the said electrode.

By means of switch I₃, electrodes G₀₂ and G₃ are raised to the polarization voltage of the substrate V_(sub). Thus, a weak surface potential φ_(sub) is created beneath G₀₂ and G₃.

At time t₁, signal V_(D) applied to diode D passes from V_(D1) to V_(D2) and then returns to V_(D1), voltages V_(D1) and V_(D2) being respectively higher and lower than the surface potential φ_(ref), beneath transfer electrode G₀₁.

The level of the charges beneath G₁, G₀₁ and D, when diode D receives voltage V_(D2) is shown by broken lines in FIG. 1a. When the diode D again receives voltage V_(D1), the level of the charges beneath electrode G₁ decreases and a reference charge quantity Q_(ref) is stored beneath electrode G₁. The surface potential beneath electrode G₁ is fixed at value V_(A2) equal to φ_(ref) by transfer electrode G₀₁.

FIG. 1b shows the various surface potentials in the substrate at time t₂. At time t₂, switches I₁, I₂ and I₃ are in the position opposite to that shown in FIG. 1a. At time t₂, switch I₁ is open and point P is left floating.

Switch I₂ is connected to the polarization voltage of substrate V_(sub). The surface potential below transfer electrode G₀₁ is therefore fixed at φ_(sub).

Switch I₃ is connected to the constant voltage V₀. The surface potential below transfer electrode G₀₂ is thus fixed at φ_(ref). The surface potential below the storage electrode G₃ is fixed, due to the thickness difference of the oxide on which the electrodes G₀₂ and G₃ are deposited at a level higher than φ_(ref).

When a charge quantity Q_(e) arrives under storage electrode G₂, the surface potential below this electrode varies and passes from V_(B1) to V_(B2). As point P is left floating, this surface potential variation is transmitted to storage electrode G₁. The surface potential below the storage electrode G₁ is fixed at level φ_(ref), by transfer electrode G₀₂. Thus, there is a transfer of a charge quantity from G₁ to below the storage electrode G₃. It is demonstrated that the charge quantity transferred beneath G₃ is equal to Q_(e) and consequently the charge quantity remaining below G₁ is equal to Q_(ref) -Q_(e).

The embodiment of the device according to the invention shown in FIG. 1a thus makes it possible to generate beneath the storage electrode G₃ a charge quantity equivalent to that arriving under electrode G₂. It is also possible to eliminate the surplus charge quantity Q_(e) by transmitting it beneath diode D. For this purpose, it is merely necessary to maintain the transfer electrode G₀₂ at voltage V_(sub) and transfer electrode G₀₁ at voltage V₀ for times t₁ and t₂.

In order that the surface potential beneath electrode G₁ is maintained constant during the arrival of the charge quantity Q_(e) beneath G₂, it is necessary that the voltages applied respectively at time t₁ to electrode G₀₁ and at time t₂ to electrode G₀₂ are equal, this applies in the case where a charge quantity Q_(e) is transferred beneath G₃ ; in the case where a charge quantity Q_(e) is transferred beneath the diode D, the voltages applied to G₀₁ at times t₁ and t₂ have to be constant.

However, the voltage V_(P) at which point P is connected at time t₁, said voltage being V_(P), can differ from the voltage V_(O) making it possible to establish the reference potential φ_(ref) beneath electrode G₀₁. When V_(P) is equal to V_(O), the device according to the invention is simplified and its accuracy increased.

FIG. 2 shows a circuit diagram for the embodiment of the invention shown in FIG. 1a. Capacitances C_(A) and C_(B) represent the oxide capacitances of electrode G₁ and G₂. Capacitances D_(A) and D_(B) represent the depletion capacitances of electrodes G₁ and G₂. In the diagram of FIG. 2, capacitances D_(A) and D_(B) are connected to earth, which constitutes the dynamic reference of the device according to the invention. A and B are the respective connection points of capacitances C_(A) and D_(A) and capacitances C_(B) and D_(B).

The charge relationships at points A, B and P are written:

at time t₁, after injecting the reference charge quantity Q_(ref) beneath G₁ :

    Sum of the charges at A=φ.sub.ref ·D.sub.A +(φ.sub.ref -V.sub.O)·C.sub.a =Q.sub.ref

    Sum of the charges at B=V.sub.B1 ·D.sub.B +(V.sub.B1 -V.sub.O)·C.sub.B =0

    Sum of the charges at P=(V.sub.O -φ.sub.ref)·C.sub.A +(V.sub.O -V.sub.B)·C.sub.B =Q.sub.p

at time t₂

    Sum of the charges at A=φ.sub.ref ·D.sub.A +(φ.sub.ref -V.sub.F)·C.sub.A =Q.sub.S

    Sum of the charges at B=V.sub.B2 ·D.sub.B +(V.sub.B2 -V.sub.F)·C.sub.B =Q.sub.e

    Sum of the charges at P=(V.sub.F -φ.sub.ref)·C.sub.A +(V.sub.F -V.sub.B2)·C.sub.B +Q.sub.P

In the above V_(S) and Q_(S) represent the voltage at point P and the charge quantity remaining beneath G₁ at time t₂ when charge quantity Q_(e) has passed beneath G₂.

The solving of the six above equations makes it possible to establish the following expression of Q_(S) : ##EQU1##

Thus, by taking the depletion capacitance D_(B) as being well below the oxide capacitances C_(A) and C_(B), as is generally the case, the following equation is proved:

    Q.sub.S =Q.sub.ref -Q.sub.e                                (1)

Subtraction has taken place between the reference charge quantity Q_(ref) and the charge quantity Q_(e) and consequently so has the generation of a charge quantity equivalent to Q_(e).

It is possible to reduce the size of the depletion capacitance D_(B) compared with oxide capacitances C_(A) and C_(B) by using an only slightly doped substrate of the order of 5·10¹⁴ cm⁻³.

It is possible to arrive at a circuit diagram for the device according to the invention and similar to that shown in FIG. 2 in the case where n storage electrodes G₂₁ to G_(2n) are electrically connected at point P to the first storage electrode G₁. The setting down of the charge equations makes it possible to establish that: ##EQU2## Q_(ei) representing the charge quantity arriving beneath an electrode G_(2i) at time t₂. Thus, the device according to the invention makes it possible to carry out the subtraction between a reference charge quantity Q_(ref) and a charge quantity ##EQU3## A charge quantity equivalent to n charge quantities Q_(e1) to Q_(en) is also generated.

As in the case where the device only has a single storage electrode G₂ (equation 1) it is only possible to use equation (2) when the depletion capacitances are very small compared with the oxide capacitances, which is generally the case.

Setting down the charge equations at point A at time t₁ before and after the injection of the reference charge quantity Q_(ref) makes it possible to obtain the following equation of Q_(ref) :

    Q.sub.ref =(φ.sub.ref -V.sub.A1)·(C.sub.A +D.sub.A)

It is therefore apparent that Q_(ref) is proportional to C_(A) +D_(A), i.e. to the surface S₁ of storage electrode G₁ for a given value V_(O). Obviously, the reference charge quantity Q_(ref) must be greater than the maximum value of ##EQU4## V_(O) and S₁ must be determined by taking account of this condition.

FIG. 3 shows another circuit diagram relative to the embodiment of the device according to the invention shown in FIG. 1a. At point P, is shown the stray capacitance C_(P), which is more particularly due to the covering of the transfer and storage electrodes and to the MOS transistor which is connected to point P for replacing switch I₁. Diode d which is connected to point A and is shown in broken line form, indicates that the device according to the invention only operates when charges are supplied and then reduction in the surface potential below electrode G₂.

The potential variations at point B and P, ΔV_(B) and ΔV_(F) are written: ##EQU5## These equations make it possible to establish the variation of the charges at point A: ##EQU6## in which Q_(S) represents the quantity of charges beneath G₁ after the arrival of Q_(S) beneath G₂.

The depletion capacitance D_(B) must obviously be negligibly small compared with the oxide capacitances for the above equation to be proved.

Thus, with regard to equation (3) it can be seen that the device according to the invention is carried out by the subtraction between Q_(ref) and Q_(e) and the generation of Q_(e), provided that the stray capacitance C_(P) is negligible compared with the oxide capacitance C_(A). The same condition must be proved in the case of a device having n capacitances G₂₁ to G_(2n) connected to G₁. 

What is claimed is:
 1. A subtraction and charge quantity generation charge transfer device ensuring the subtraction of n charge quantities from a reference charge quantity and the generation of a charge quantity equivalent to the n charge quantities and comprising a semiconductor substrate in which charges are stored, wherein it comprises a first storage electrode electrically connected at a point to n other storage electrodes, means ensuring the injection beneath the first storage electrode of the reference charge quantity prior to the arrival of the charges beneath the n storage electrodes and when the said point is at a constant voltage and means which maintain a constant surface potential beneath the first storage electrode during the arrival of the charges beneath the n storage electrodes and when the said point is left floating.
 2. A device according to claim 1, wherein the means ensuring the injection of the reference charge quantity beneath the first storage electrode are constituted by two charge transfer electrodes surrounding the first storage electrode, the two transfer electrodes being raised to different voltages, and by a charge injection diode implanted in the semiconductor substrate following the transfer electrode raised to the higher voltage, said injection diode being successively raised to a voltage which is lower and then higher than the surface potential beneath the transfer voltage raised to the higher voltage.
 3. A device according to claim 2, wherein the means which maintain a constant surface potential beneath the first storage electrode are constituted by the two charge transfer electrodes surrounding the first storage electrode, one of the transfer electrodes being raised to the higher voltage and the other transfer electrode being raised to a lower voltage, and by a storage electrode or an injection diode following the transfer electrode raised to the higher voltage, the surface potential beneath the storage electrode or beneath the diode being higher than the surface potential beneath the transfer electrode raised to the higher voltage.
 4. A device according to claim 3, wherein the constant voltage of the said point where the first storage electrode is connected to n other storage electrodes is identical to the voltage of the said transfer electrode raised to the higher voltage.
 5. A device according to claim 1, wherein the semiconductor substrate is weakly doped at approximately 5·10¹⁴ cm⁻³. 